There are many injectors on the market where the aim is to have high degrees of functionality and automatic features, such as in connection with penetration, injection, setting of dose, priming and covering of the needle after use. At the same time there is a demand on robustness, repeatability and reliability regarding the function, which sometimes might be difficult to meet when dealing with complex multi functions involving many interacting components. When there further are demands on low production costs, especially for devices that are to be used only once, the picture becomes even more complex.
There are in the patent literature numerous solutions to injection devices, the bulk of which never enter the market due to that they do not meet the demands in one way or the other. There is therefore a continuous search for solutions that provide the desired functions that at the same time fulfill the functional and/or economical demands.
Many devices having multi-functions that work in sequence, such as for example penetration, followed by injection, followed by withdrawal, have a subsequent sequence triggered at the end of a previous sequence, for example when the needle has reached full penetration depth, the injection sequence is triggered.
Safety margins have been implemented in the devices having multi-functions that work in sequence in order to ensure that each sequence is effective, e.g., in WO-A1-2006057604. Mostly the safety margins are overestimated in the devices, leading to medicament spill after the needle withdrawal, which also results in inaccurate delivered doses.
In EP-A-0 516 473, a retraction mechanism has been proposed wherein, at the point at which a plunger rod reaches the end of a bore in a syringe, a portion of a coupling instantaneously collapses in length as a retraction spring retracts a needle. This proposal suffers the problem that, due to the tolerances of the various components, it cannot be assured that the retraction mechanism will enable retraction of the needle at precisely the moment at which the plunger rod reaches the end of the bore. Therefore, either the mechanism retracts the needle before the plunger rod reaches the end of the bore, such that the syringe is not emptied, or the plunger rod reaches the end of the bore before the mechanism has moved sufficiently far to retract.
Although this problem has been recognized in U.S. Pat. No. 6,159,181 and EP-B1-0 996 473, the proposed solutions have been to provide a user actuated withdrawal mechanism rather than an automatic one and this is considered to be undesirable due to the risk of removing the injector from the injection site before the user actuated withdrawal mechanism has been activated. Then there exists the dangers of having an extended and possibly contaminated needle exposed around the user.
It is an object of the present invention to provide an injection device having an active triggering withdrawal mechanism, which is relatively simple and of low cost and which overcomes or at least reduces the problems identified above.